The nature of the electronic coupling between donors and acceptors in electron transfer processes is an important consideration in determining the rates and therefore the yields of such reactions. It is likely that the physical coupling between donors and acceptors also plays a pertinent role in the electronic coupling in such systems. In order to investigate the effect of the physical coupling on the electronic coupling, hence the electron transfer rate constants, we are studying compounds in which the driving force for electron transfer reaction remains essentially constant but the physical coupling between electron transfer partners differs. Specifically, we have synthesized a series of metallo-porphyrin systems which consist of an electron donor and acceptor pair linked by a series of phenyl-ring spacers which are either covalently linked to the porphyrin ring or coordinated axially to the metal contained in the porphyrin ring. The rates for forward and reverse electron transfer have been measured by femtosecond pump-probe spectroscopy and TCSPC (time correlated single photon counting). We have concluded that there is, indeed a difference in the electron transfer rate constants which reflects the physical coupling of the donor and acceptor moieties.